Fabrication techniques for microelectronic devices require that small, exacting changes be made in very thin films of material. In order to produce these types of thin films accurately and consistently, an in situ measurement technique is needed to monitor the film thickness during the different process steps which effectuate thickness changes. Various measurement techniques have previously been developed to measure films, but none of which meet the requirement of measuring thin films (i.e. films with thicknesses on the order of one monolayer).
Heretofore, the best measurement technique for each application or process has been dependent upon the film type, the thickness of the film, and the accuracy desired. These criteria include such properties as film thickness, film transparency, thickness uniformity, film and substrate smoothness, film and substrate optical properties, and film and substrate size. Various types of optical interference phenomena, or optical interferometry, have been found to be most useful for the measurement of thin films, which are generally on the order of a wavelength of light. These optical interferometry measurement systems may be classified into two categories, those that measure transparent films and those that measure opaque films. It has been found that the most widely used thin film optical interferometry techniques measure transparent films on silicon substrates. However, advancements in semiconductor technology have resulted in a specific need for measuring very thin film opaque materials, such as metals and stacked layers, on a variety of substrates or media.
Prior techniques for monitoring opaque films typically rely on the use of 2-beam interferometry to extract the etched rate of the material. In such arrangements, a single wavefront of light is split into two beams, one of which, strikes a non-etching surface which serves as a reference while the other beam strikes the surface being etched which is moving away from the reference. The change in path length thus causes a phase shift which produces a sinusoidal type signal after the two beams are recombined at a photodetector. An example of this types of system can be found in U.S. Pat. No. 4,147,435 (Habegger, et al.). These types of systems, however, suffer from inherent poor thickness control since the total etch depth is on the same order of magnitude as the resolution of the system itself.
More recent attempts to solve the deficiencies of the two beam systems involve the use of monitor wafers to provide reference reflected signals which are compared to reflections off of the surface under process. An example of this type of system is disclosed in commonly owned U.S. Pat. No. 4,367,044 (Booth Jr., et al.).
The optical interferometry systems disclosed in Habegger, et al. and Booth Jr., et al., however, are impractical for present VLSI fabrication technology, because of inadequate resolution, and difficulties inherent in aligning the systems and maintaining the alignment.
A system for accurately measuring opaque film thickness in situ during etching and deposition processes which overcomes the deficiencies of the prior art is therefore highly desirable.